Quantifying the Thermo-Mechanical Response and Strain-Rate Effects in Magnesium Microcrystals

非技术摘要：该项目将重点量化镁及其合金在高温和不同变形速率下的变形机制。减少化石燃料动力车辆的二氧化碳排放的一种方法是通过减少车辆结构重量来提高其燃料效率。为此，镁合金表现出有利的强度重量比，并提供铝合金和高强度钢的有效替代品。因此，在许多汽车、航空航天和国防应用中利用镁合金的兴趣越来越大。然而，目前，镁合金广泛生产和使用的限制因素之一是其在室温下的低成形性，这导致许多技术和经济约束。这主要是由于镁合金在室温下的低延展性。因此，大多数镁片是在低于其再结晶极限（~400ºC）的高温下形成的。另一个挑战是Mg的高应变率敏感性，这需要低速成形工艺。因此，该奖项将支持通过一套最先进的多长度尺度实验和模拟技术从根本上确定变形机制的研究。该项目的结果预计将有助于开发新的镁合金，在更广泛的温度和应变速率范围内表现出更好的成形性和更高的延展性和韧性。在该项目中整合研究、教育和外联还将：（1）提高巴尔的摩一所以非洲裔美国人为主的小学的STEM成绩，该小学目前在马里兰州的学校中排名低于92%;（2）通过机械和材料研究实习，让当地一所历史悠久的黑人大学的学生参与进来;以及（3）开发一个教育组合，加强本科生和研究生在最先进的多尺度建模和微尺度实验基础方面的知识。技术摘要：本研究的主要目的是从根本上确定Mg和AZ 31微晶的应变速率耦合效应和热-机械响应（在大块单晶中制造）通过耦合新颖的原位扫描电子显微镜高温实验，和新颖的大规模三维离散位错动力学模拟，其考虑位错-孪晶界相互作用。该研究围绕解决四个具有挑战性的目标：（1）量化Mg和AZ 31微晶在30至400ºC范围内的热机械性能和变形机制;（2）确定在Mg熔化温度的30%下观察到的异常硬化响应的起源;（3）量化了在1 e-4 ~ 0.1s范围内不同温度下应变速率对变形机制的影响;以及（4）生成温度、应变速率、晶体尺寸和强度之间的相关性的四维模型。

NON-TECHNICAL ABSTRACT:This project will focus on quantifying the deformation mechanisms that dominate at elevated temperatures and different deformation rates in magnesium (Mg) and its alloys. One approach to reduce carbon dioxide emission from fossil-fuel powered vehicles is to increase their fuel efficiency by reducing the vehicle structural weight. To that end, Mg alloys demonstrate a favorable strength-to-weight ratio and provide valid alternatives to aluminum alloys and high strength steels. Thus, there is a growing interest in utilizing Mg alloys in many automotive, aerospace and defense applications. However, at present, one of the limiting factors for the wide production and use of Mg alloys is their low formability at room temperature, which leads to many technological and economical constraints. This is mainly due to the low ductility of Mg alloys at room temperature. As such, most Mg sheets are formed at elevated temperatures below their recrystallization limit (~400ºC). Another challenge is the high strain-rate sensitivity of Mg, which necessitates low speed forming processes. This award will thus supports research to fundamentally identify the deformation mechanisms through a set of state-of-the-art multi-length scale experimental and simulation techniques. The results of this project are expected to assist in the process of developing new Mg alloys that demonstrate improved formability and enhanced ductility and toughness at a wider range of temperatures and strain rates. The integration of research, education and outreach in this project will also: (1) improve STEM achievement in a predominately African American elementary school in Baltimore, currently ranked below 92% of schools in Maryland; (2) involve under-represented students from a local historically black college through internships on research in mechanics and materials; and (3) develop an education portfolio that intensifies the knowledge of undergraduate and graduate students in fundamentals of state-of-the-art multiscale modeling and micro-scale experiments.TECHNICAL ABSTRACT:The primary research objectives of this research are to fundamentally identify the coupled strain rate effect and thermo-mechanical response of Mg and AZ31 microcrystals (fabricated in bulk single crystals) through coupled novel in situ scanning electron microscopy elevated-temperature experiments, and novel large scale three-dimensional discrete dislocation dynamics simulations that account for dislocation-twin boundary interactions. The research hinges around addressing four challenging objectives: (1) Quantify the thermo-mechanical properties and deformation mechanisms in the range of 30 to 400ºC in Mg and AZ31 microcrystals; (2) Identify the origins of the anomalous hardening response observed at 30% of the melting temperature of Mg; (3) Quantify the strain rate effects at different temperatures on the deformation mechanisms in the range of 1e-4 to 0.1 s; and (4) Generate a four dimensional model of the correlation between temperature, strain rate, crystal size, and strength.